When microbes penetrate the skin and mucous membranes or bypass the antimicrobial substances in blood, the next line of defense consists of natural killer cells and phagocytes. We will first consider natural killer cells.
In addition to B cells and T cells, which participate in immune responses, there is another population of lymphocytes that destroys intruders. These lymphocytes, called natural killer (NK) cells, have the ability to kill a wide variety of infectious microbes plus certain spontaneously arising tumor cells.
natural killer cells are present in the spleen, lymph nodes, bone marrow and blood.
Gamma interferon is released by natural killer cells and also stimulates their cytolytic activity.
Exactly how natural killer cells deliver their 'kiss of death' is not completely clear. They may release perforins, chemicals that cause cytolysis of the microbe. Or they may bind to a target cell and inflict damage by direct contact. How natural killer cells recognize their targets is also an unsettled question.
They probably attack cells that do not display proper identify markers called major histocompatibility complex (MHC) antigens. natural killer cells are defective or decreased in number in some cancer patients and in patients with AIDS.
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